Application of a multicoloring technique by a color conversion method to liquid crystal displays, organic electroluminescence (EL) displays, lighting apparatuses, and the like is being energetically studied. Color conversion means converting light emission from a light-emitting body into light with a longer wavelength and means converting blue light emission into green or red light emission, for example.
A composition having this color conversion function (a color conversion composition) is formed into a sheet form and is combined with a blue light source, for example, whereby the three primary colors of blue, green, and red can be extracted, that is, white light can be extracted from the blue light source. A white light source obtained by combining the blue light source and the sheet having the color conversion function with each other forms a backlight unit, and this backlight unit, a liquid crystal drive part, and color filters are combined with each other, whereby a full-color display can be produced. Without the liquid crystal drive part, the residual part can be used as a white light source as it is, which can be used as the white light source such as LED lighting.
Improvement in color reproducibility is a problem in liquid crystal displays using the color conversion method. Narrowing the full width at half maximum of the respective emission spectra of blue, green, and red of the backlight unit to increase the color purity of each of blue, green, and red is effective in improving color reproducibility. To solve this problem, developed is a technique that uses quantum dots formed of inorganic semiconductor fine particles as a component of the color conversion composition (refer to Patent Literature 1, for example). Although the technique using the quantum dots indeed is narrow in the full width at half maximum of green and red emission spectra to improve color reproducibility, the quantum dots are vulnerable to heat and water and oxygen in the air and are thus deficient in durability on the other hand.
Also developed is a technique that uses a light-emitting material formed of an organic substance as a component of the color conversion composition in place of the quantum dots. Disclosed as examples of the technique that uses an organic light-emitting material as the component of the color conversion composition are one that uses a coumarin derivative (refer to Patent Literature 2, for example), one that uses a pyridine-phthalimide condensation product (refer to Patent Literature 3, for example), one that uses a rhodamine derivative (refer to Patent Literature 4, for example), and one that uses a pyrromethene derivative (refer to Patent Literature 5, for example).